1. Field of the Invention
The present invention relates to a spacer for maintaining a cell gap between an upper substrate and a lower substrate of a liquid crystal display device, and in particular to a liquid crystal display device using both a ball spacer and a patterned spacer having a patterned shape.
2. Description of the Related Art
A liquid crystal display device commonly includes a lower substrate upon which a plurality of switching devices are arranged in a matrix configuration and an upper substrate upon which a plurality of color resins are arranged in a matrix configuration for displaying image data. The plurality of switching devices commonly includes a thin film transistor (TFT) array substrate having a plurality of gate lines arranged in rows along a horizontal direction, a plurality of data lines arranged in columns along a vertical direction so as to cross the gate lines, and a TFT is commonly formed at each cross point of the gate and data lines to function as a switching device.
The color filter substrate includes a black matrix for shielding unwanted light that proceeds directly from a lower portion of the liquid crystal display, a sub-color filter having red, green, and blue colors, and a common electrode for supplying an electric field to liquid crystal molecules of a liquid crystal material. In addition, a cell gap between the color filter substrate and the TFT array substrate is commonly maintained by a spacer, and the liquid crystal material fills the cell gap.
FIG. 1 is a flow chart of a fabrication method of a liquid crystal display device according to the related art. In FIG. 1, the fabrication method includes preparing a color filter substrate, forming an alignment layer on a TFT array substrate, rubbing the alignment layer for freely aligning subsequently-formed liquid crystal of a liquid crystal material, forming a seal pattern, scattering spacers for maintaining a uniform cell gap between the TFT array substrate and the color filter substrate, bonding the TFT array and color filter substrates together, cutting the bonded substrate into a plurality of unit cells, and injecting the liquid crystal material into each of the unit cells.
During a first step, a transparent substrate is prepared, and a plurality of switching devices and gate and data lines are arranged on the transparent substrate in a matrix configuration. In addition, a pixel electrode corresponding to each of the switching devices is formed.
During the second step, the alignment layer is formed on the lower substrate using a coating process for coating a polymer substance, and a rubbing process for alignment of liquid crystal molecules of the liquid crystal material is performed. The alignment layer is a thin polymer film, such as a polyimide, and is uniformly deposited onto the lower substrate. The rubbing process includes rubbing the alignment layer along a certain direction with a fabric, whereby the liquid crystal molecules are aligned along the certain direction according to the rubbing direction. The rubbing process is important to determine the initial alignment of the liquid crystal molecules so that the liquid crystal display device can perform normally and have uniform display characteristics.
During a third step, the seal pattern is formed to maintain a cell gap between the upper and lower substrates in which the liquid crystal material is injected. In addition, the seal pattern prevents the injected liquid crystal material from leaking out between the TFT array and color filter substrates. The seal pattern commonly includes a thermosetting resin and is formed as a pattern along a perimeter region of an active region of the lower substrate using a screen printing method.
During the fourth step, the spacers are formed to have a certain size to maintain the uniform cell gap between the TFT array and color filter substrates, and are scattered uniformly onto the color filter substrate. The scattering method can be commonly divided into a wet scattering method for mixing the spacers in an alcohol solution, and spraying the mixture, and a dry scattering method for scattering only the spacers. In addition, the dry scattering method is commonly divided into an electrostatic scattering method using static electricity and an anti-static scattering method using gas pressure. The anti-static scattering method is mostly used for a liquid crystal cell structure having weak static electricity.
During the fifth step, after the spacer scattering is completed, the bonding process for attaching the color filter substrate and the TFT array substrate is performed. A uniform cell gap generated during the bonding process is determined by an error margin between the upper color filter and lower substrates for individually designing liquid crystal display devices with an accuracy of several μm. When the uniform cell gap exceeds the error margin, light leaks from the device, thereby decreasing image quality display of the liquid crystal cell.
During the sixth step, the bonded lower and upper glass substrates are cut into a plurality of unit cells. During prior cutting processes, liquid crystal material is simultaneously injected into the plurality of cells, and the cutting is performed. However, with the presently increased size of liquid crystal display devices, the liquid crystal material is injected after the cutting is performed.
During the seventh step, the liquid crystal material is injected into each of the plurality of unit cells. Each of the unit cells has a cell gap of several μm per several hundreds of cm2. Accordingly, a vacuum injection method for efficiently injecting the liquid crystal material into the liquid crystal cell is commonly used. The vacuum injection method uses a pressure difference between interior and exterior of the liquid crystal cell through an injection hole in the seal pattern. After the injection of the liquid crystal material, excess liquid crystal material on a liquid crystal injection hole is removed and the injection is sealed. Accordingly, a panel of the liquid crystal display device is fabricated.
However, during the vacuum injection process, an excessive amount of the liquid crystal material is injected into the liquid crystal display panel. Accordingly, shifting of the liquid crystal material, caused by gravity and the like, may occur in subsequent testing processes. Thus, a fabrication process for injecting limited amounts of liquid crystal material into the liquid crystal display panel is required. In addition, a sealing method for removing any of the excessive liquid crystal material has been developed by applying mechanical pressure onto the liquid crystal display panel, or by using high gas pressure gas.
FIG. 2A is a schematic view of a ball spacer scattering method in according to the related art. In FIG. 2A, according to size increase of a panel, spacers 202 are uniformly scattered on an entire surface of a substrate 201 in order to maintain a uniform cell gap. However, since an alignment layer of the substrate 201 may be damaged, scattering of the spacers 202 must be performed carefully.
In response to demands for high display capacity and high display quality, providing the spacers not only functions to maintain a uniform cell gap but also functions to prevent color tone variations according to temperature variations or movement of the spacers or creation of voids. Accordingly, the spacers are commonly divided into glass spacers, which are fabricated by discharging non-alkali glass, and plastic spacers. In comparison with hard glass ball spacers, the plastic ball spacers are unstable to maintain the uniform cell gap. However, performance of the plastic ball spacers is dependent upon elastic body variables directly related to a load applied to the plastic ball spacers. Thus, it is appropriate that a substance for forming the plastic ball spacers can maintain a minute cell thickness.
By using a material for the plastic spacers that has a thermal coefficient of expansion close to the thermal coefficient of expansion of the liquid crystal material, it is possible to prevent the plastic spacers from migrating within the cell gap when the unit cells are exposed to relatively high temperatures. Conversely, the plastic spacers prevent creation of voids when the unit cells are exposed to relatively low temperatures.
FIG. 2B is a cross sectional view of a liquid crystal display panel in according to the related art. In FIG. 2B, a color filter substrate 203 is overlapped with a TFT array substrate 201 to contact the plastic ball spacers 202, and the two substrates 201 and 203 are bonded together by the seal pattern 204. Accordingly, since the plastic ball spacers 202 have a thermal coefficient of expansion similar to that of the liquid crystal material, the unit cell may be operated under varying temperature conditions. However, by spraying the plastic ball spacers 202 onto the TFT array substrate 201 having an alignment layer, it is impossible to adjust the scattering position of the plastic ball spacers 202. Accordingly, the scattering density is not uniform across an entire surface of the lower TFT array substrate 201. Thus, when the plastic ball spacers 202 are scattered onto the pixel region of the TFT array substrate 201, contrast of the liquid crystal display device may decrease.
In addition, since the scattering density of the plastic ball spacers 202 varies according to positions of the unit cells, external impact and corresponding impact absorption degrees differ according to positions of the unit cells. Accordingly, vibration generation ratios vary. Thus, the individual liquid crystal display devices will have different trembles according to the positions of the unit cells such that a ripple phenomenon is displayed by the device.
In order to solve the above-described problem, a patterned spacer forming method capable of scattering spacers formed of photosensitive resin at certain positions on the substrate has been developed, wherein a shape of the patterned spacer is controlled. During a fabrication process of the liquid crystal display device, formation of the patterned spacers is nearly the same as the formation of the ball spacers except for an additional spacer patterning process. Accordingly, formation of the spacers allows for position adjustment and patterning of the spacers.
It is possible to form the patterned spacers anywhere on the upper substrate or the lower substrate. However, in general, the patterned spacers are formed on the upper substrate. The upper substrate and the lower substrate are fabricated separately and are bonded together during a subsequent process. Accordingly, by forming the spacers on the upper substrate using a comparatively simple process, it is possible to simultaneously fabricate the two substrates.
The step of forming the patterned spacers on the upper substrate includes preparing the substrate, forming a black matrix for shielding unnecessary light on the substrate, forming a color filter made of red, green, and blue color resins, forming an overcoat layer for plating the surface of the color filter layer; forming a common electrode for supplying an electric field to a liquid crystal material formed on the overcoat layer, forming patterned spacers made of photosensitive resin on the common electrode, and forming an alignment layer made of a polyimide group material for alignment of the liquid crystal material on the patterned spacers.
FIGS. 3A and 3B are cross sectional views of forming patterned spacers according to the related art. In FIG. 3A, the patterned spacers are formed using a photolithographic method, wherein a photosensitive resin 306 is deposited on the common electrode 305. Then, a mask 307, which includes a spacer pattern, is positioned to cover the photosensitive resin 306, and ultraviolet light is irradiated through the spacer pattern onto the photosensitive resin 306. Accordingly, subsequent processing provides that regions of the photosensitive resin 306 that have been exposed to the ultraviolet light remains while other regions of the photosensitive resin 306 not exposed to the ultraviolet light are removed, thereby forming the patterned spacers on the common electrode 305. Thus, by providing the spacers on specific portions of the common electrode 305, it is possible to scatter the spacer on the black matrix region except where the pixel regions are formed. Accordingly, contrast ratio and aperture ratio may not be reduced.
However, at high operating temperatures, use of the patterned spacers causes gravity inferiority in which the liquid crystal material is inclined to migrate along the gravity direction. Accordingly, the liquid crystal material may pool at a bottom most region of the liquid crystal display device. However, thermal expansion of the patterned spacers is smaller the thermal expansion of the liquid crystal material and glass used to form the upper and lower substrates. Accordingly, when the liquid crystal display device is tested at relatively high temperatures, a gap is created between the glass substrate and the patterned spacers due to the difference in the thermal expansion of the patterned spacer material and the glass substrates. Thus, the liquid crystal material leaks through the gap and migrates toward the gravity direction.
In FIG. 3B, upper and lower substrates 32 and 31 are bonded together using a sealant 33. Patterned spacers 34 are positioned to maintain a uniform gap between the upper and lower substrates 32 and 31, and a liquid crystal material 35 is injected into the gap between the upper and lower substrates 32 and 31 through an injection hole (not shown). Accordingly, an increase in the amount of the liquid crystal material 35 filling the gap results in increasing the effects of gravity inferiority. Thus, excessive amounts of the injected liquid crystal material are removed by applying pressure on the both exterior surfaces of the liquid crystal display panel with a flat metal plate using a mechanical method after injecting the liquid crystal material 35 into the gap and before encapsulating the injection hole (not shown). Alternatively, the excessive amounts of the injected liquid crystal material may be removed by injecting gas into a chamber in which the liquid crystal material injection process is performed to form a high pressure state on both of the exterior surfaces of the liquid crystal display panel using a sealing method.
By using the sealing method, small amounts of the liquid crystal material are injected into the liquid crystal display panel. Accordingly, negative pressure imparted onto the glass substrates along an interior direction is formed between the upper and lower substrates of the liquid crystal display panel. Thus, it is possible to prevent the liquid crystal material from migrating toward the gravity direction at high temperatures. However, although the sealing method is used, it is impossible to completely prevent the gravity inferiority between the patterned spacers.